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Order too little concrete and a pour stops halfway with no way to catch up cleanly; order too much and you’ve paid for waste. This calculator turns the dimensions of a slab or footing into the volume you need — in cubic yards and cubic metres — and the number of pre-mix bags for smaller jobs.
Choose imperial or metric, enter length, width and thickness, and get the volume plus bag counts for 80, 60 and 40 lb bags.
How is it calculated?
Volume is length × width × thickness
The maths is simple; the trap is units, because thickness is measured in different units from length. This tool keeps them straight:
| System | Length & width | Thickness | Result |
|---|---|---|---|
| Imperial | feet | inches | cubic yards |
| Metric | metres | centimetres | cubic metres |
One cubic yard = 27 cubic feet = about 0.765 cubic metres.
How many bags?
Bagged concrete is sold by set volume yield. The calculator divides your total by each bag’s yield and rounds up:
| Bag | Approx. yield |
|---|---|
| 80 lb | 0.60 ft³ |
| 60 lb | 0.45 ft³ |
| 40 lb | 0.30 ft³ |
Bags suit footings, posts and small pads. For anything approaching a cubic yard, ready-mix delivery is usually cheaper and faster than mixing dozens of bags by hand.
Add a waste allowance
Real pours lose a little to spillage, uneven sub-base and over-excavation. Add roughly 5–10% to the calculated amount, and round up when ordering ready-mix — running short mid-pour is far more costly than a small surplus.
Worked example
A patio slab 10 ft long, 10 ft wide and 4 inches thick works out to 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards (about 0.94 cubic metres). In 80 lb bags that’s ⌈33.3 ÷ 0.60⌉ = 56 bags — clearly a job for ready-mix rather than bags. Adding a 10% waste margin, you’d order about 1.36 cubic yards.
FAQ
How do I calculate how much concrete I need?+
Multiply length × width × thickness to get the volume, keeping units consistent. This tool does it for you and converts to cubic yards and cubic metres, handling the fact that thickness is entered in inches (imperial) or centimetres (metric) while length and width are in feet or metres.
How many bags of concrete are in a cubic yard?+
A cubic yard is 27 cubic feet. At 0.60 ft³ per 80 lb bag that’s 45 bags; at 0.45 ft³ per 60 lb bag it’s 60 bags; at 0.30 ft³ per 40 lb bag it’s 90 bags. That’s why ready-mix is preferred once you approach a full yard.
Should I order extra concrete?+
Yes — add about 5–10% for spillage, an uneven sub-base and slight over-digging. Running out mid-pour creates a cold joint and a weak spot, so a small planned surplus is much cheaper than coming up short.
When should I use ready-mix instead of bags?+
Bags make sense for footings, fence posts and small pads up to roughly a quarter cubic yard. Beyond that, mixing dozens of bags by hand is slow and inconsistent; a ready-mix delivery is usually cheaper, stronger and far quicker.
What thickness should a slab be?+
It depends on the load: patios and walkways are commonly around 4 inches (10 cm), while driveways and areas taking vehicles are often 5–6 inches (13–15 cm). Follow local codes and, for structural work, an engineer’s specification.